When soybean is processed into food, there is an undesirable odor. This is called an unpleasant odor. The known causative substances for this unpleasant odor include aliphatic carbonyl compounds such as acetaldehyde, acetone and n-hexanal; aromatic carbonyl compounds such as benzaldehyde and protocatechu aldehyde; volatile fatty acids such as acetic acid, propionic acid, n-valeric acid, isovaleric acid, n-caproic acid, isocaproic acid, n-caprylic acid and pelargonic acid; volatile amines such as ammonia, monomethylamine, dimethylamine, trimethylamine and piperidine; volatile aliphatic alcohols such as methanol, ethanol, 2-pentanol, isopentanol, n-hexanol and n-heptanol; phenol acids such as vanillic acid, ferulic acid, gentisic acid, salicylic acid and hydroxybenzoic acid, etc. Of these, particularly, n-hexanal has a specific unpleasant odor similar to grassy smell.
The n-hexanal is a substance generating an unpleasant odor generated by oxidation of soybean oil by an enzyme, lipoxygenase, which exists in a soybean seed. That is to say, lipoxygenase is an enzyme, which adds molecular oxygen to unsaturated fatty acids having a cis or cis-1,4-pentadiene structure to generate hydro-peroxide. This enzyme exists broadly in the plant kingdom, and especially, a soybean seed contains a large amount of lipoxygenase. Lipoxygenase not only deteriorates soybean oil, but also its oxide generates n-hexanal, a causative substance for a grassy, unpleasant odor. This often becomes a cause for which a soybean is shunned as a food material.
Three types of lipoxygenases exist in a soybean seed, and each of them has a different optimal pH, substrate specificity, thermostability, etc. Any of these lipoxygenases is associated with generation of an unpleasant odor in a processing step of soybeans, but a soybean which lacks lipoxygenase has already been developed (Japan J. Breed., vol. 41, p.507, 1991).
To control oxidation of soybean oil or generation of an unpleasant odor by lipoxygenase, inactivation of this enzyme by a heat processing has mainly been carried out (Lebensm Wiss Technol., vol. 26, p.215, 1993). Moreover, several methods such as steaming (J. Food Process Preserv., vol. 10, p.177, 1986), extraction with an organic solvent (Report of Shikoku Agricultural Experiment Station, vol. 61, p. 31, 1997), processing by infrared radiation (Lebensm Wiss Technol., vol. 15, p. 139, 1982), and microwave processing (J. Am. Oil Chem. Soc., vol. 68, p. 744, 1991), have been attempted to eliminate the unpleasant odor that is generated.
In addition to the unpleasant odor, the unpleasant taste of soybean food is a factor preventing the widespread use of the food. After eating or drinking soybean food, we often feel irritated over the whole area of the mouth cavity and the throat. This irritating taste is similar to a slightly astringent taste. Such an unpleasant taste is felt especially after drinking soybean milk, and this taste is caused by reaction of a taste substance with a protein-like substance on the mucous membrane to generate slight paralysis of the nerve. This unpleasant taste is also called an astringent taste.
The ingredients causing such an astringent taste include polyphenols, oxidized phospholipid, oxidized fatty acid, saponin, isoflavone or the like, and in particular, saponin has a strong astringent taste.
Examples of known saponins include group A saponin, group B saponin, group E saponin, and aglycons thereof such as soyasapogenol A, soyasapogenol B and soyasapogenol E.
Since group A acetyl saponin and soyasapogenol A, which have the strongest astringent taste, are located in the hypocotyl of a soybean, that taste can be reduced by the process of peeling a soybean and eliminating the hypocotyl. However, it is difficult to completely eliminate the hypocotyl, and also there is a disadvantage that isoflavone, group B saponin and group E saponin that are functional ingredients in a soybean fruit body are lost simultaneously in the process.